1. Field of the Invention
This invention relates to liquid crystal materials and more specifically to mixtures of nematic liquid crystal materials from different chemical classes.
2. Description of the Prior Art
Recently there has been widespread interest in the class of materials known as liquid crystals. Generally, liquid crystals exhibit some physical properties typically associated with liquids but also exhibit other physical properties typically associated with crystalline solids. For example, their viscosities are ordinarily similar to those of liquids whereas their optical properties are similar to those of crystalline solids.
Liquid crystals are known to appear in at least three different mesomorphic forms: the smectic, cholesteric and nematic forms. Nematic liquid crystal materials assume the characteristics of a thread-like texture when a thin section of the material is viewed between crossed polarizers. One type of nematic liquid crystal is normally transparent to light in the absence of an electric field but scatters light in the presence of an electric potential applied thereacross. This effect has been termed "dynamic scattering" and has become very useful in many electrooptic display devices. Another type of nematic liquid crystal has the property of altering the polarization of light under an applied electric field; this type is known as "field effect" materials and has a positive dielectric anisotropy as opposed to the negative dielectric anisotropy of dynamic scattering materials.
Nematic liquid crystal materials have traditionally exhibited high mesomorphic transition temperatures which sometimes made it necessary to add external heating apparatus to displays to maintain materials in their mesomorphic range. Another serious problem with nematic materials has been the very narrow mesomorphic temperature range which they exhibit. Previous attempts have been made to broaden the mesomorphic temperature range of nematic liquid crystal materials, but heretofore, none have been completely satisfactory. Attempts to lower the freezing point of such materials by mixing closely related compounds have not been very successful because each successive compound added tends to have less effect on the freezing point than the previously added compounds. In addition, there is often a concomitant lowering of the isotropic transition temperature which severely limits high temperature operation and tends to offset any gain achieved by lowering the freezing point.
U.S. Pat. No. 3,716,289 discloses an attempt to broaden the temperature range of nematic liquid crystal materials by combining such materials with smectic liquid crystal materials, and in some cases with smectic and cholesteric materials. While such mixtures have improved the mesomorphic temperature range somewhat, they have not resulted in greatly widened ranges.
U.S. Pat. No. 3,779,751 teaches the mixing of cholesteric liquid crystal materials with particular combinations of Schiff's bases and also teaches that various Schiff's base liquid crystal materials can be mixed to achieve wider mesomorphic temperature ranges. In spite of this, these compositions have a limited reduction of their freezing point and have a tendency to form smectic compositions over a large portion of their concentration range.
U.S. Pat. No. 3,781,088 discloses the mixing of nematic liquid crystal materials with cholesteric compounds. U.S. Pat. No. 3,655,270 discloses ternary nematic liquid crystal compositions containing three Schiff's bases to broaden the mesomorphic temperature range. U.S. Pat. No. 3,792,915 discloses that mixtures of certain asymmetric, optically active, p-alkoxybenzylidene-p'-aminobenzonitrile compounds with nematic liquid crystals formed from cholesteric liquid crystals which, upon application of an electric field, form a nematic liquid crystal.
A mixture claimed to have a wide nematic temperature range is disclosed in U.S. Pat. No. 3,655,270 to Creagh. This mixture contains three ingredients, which are: (1) 4-ethoxybenzylidene-4'-n-butylaniline (EBBA), a Schiff's base; (2) 4-methoxybenzilidene-4'-aminophenylbutyrate, a Schiff's base ester; and (3) bis(4'-n-octyloxybenzal)-2-chloro-1,4-phenylenediamine (OBCPD), a triphenyl compound capable of raising the isotropic transition temperature. A number of mixtures of these three ingredients is presented in Table I, col. 6 of this patent wherein the lowest freezing point obtained is -15.degree. C. For many applications, this is insufficient.